Optical coherent receivers require the signal has the same polarization status as the reference light beam. However, the current optical hybrid cannot control the polarization status neatly. Therefore, an additional polarization controller is needed to ensure the polarization status.
Optical hybrid has traditionally been constructed by using two 50/50-beam splitters, two beam combiners and one 90 degree phase shifter. This optical hybrid can be implemented using all-fiber or planar waveguide technology. But the two approaches have their own disadvantages. The two approaches need precise temperature control circuits to sustain precise difference of optical path length, and thus sustain an accurate output light phase. In addition, the fiber based instruments are huge in size and are unstable to mechanical shock and vibration. The waveguide based products encounter problems from high insertion loss, high polarization dependence and manufacture capacity. The waveguide products are also not flexible for customization and require substantial capital resources to set up.